Soil Resistivity Testing Services

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Liberty Consulting Services has conducted extensive electrical soil resistivity testing for many years, in some of the most difficult soil conditions Australia has to offer. During this time, LCS has developed reliable techniques to provide accurate test data so that a low margin of error is able to be obtained for earth system designs using the provided test measurements. The use of adequately powered test equipment to suit the conditions is crucial to performing meaningful electrical soil resistivity testing.

Why soil resistivity testing is so important

Soil resistivity is the basis upon which earth grids are designed so that electrical parameters are determined and modelled (e.g. grid resistance, and mesh voltages due to injected fault currents). This is a crucial step in designing and assessing any earthing system.

Soil is rarely homogeneous, and is commonly made up of horizontal layers which need to be identified so that the correct placement of earthing conductors can be determined during the design process. The use of uniform or “averaged” tested resistivity values can lead to significant errors in earthing designs.

Qualified LCS engineers perform testing so that difficult conditions encountered in the field can be handled on site, and re-testing suspect data points is done as required. This ensures the provision of accurate test data to the designing engineer(s).

These Standards provide more detail on accepted soil resistivity test methods:

ENA EG-0 Power System Earthing Guide, (Section 7 – especially 7.2.3)
ENA EG-1 Substation Earthing Guide (Section 5.2 – especially 5.2.2)
AS/NZS 1768 Lightning protection (Appendix C.10 – especially C10.1)
IEEE Std 80 IEEE Guide for Safety in AC Substation Grounding (Sections 13.3 & 13.4)
IEEE Std 81 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System (Section 7)

Soil electrical characteristics

The key determinant of error in the earthing design process is the inaccuracy or variation of the soil resistivity input to the design.

Therefore it is important to understand electric current flow in non-uniform, multi-layered soils, and how to accurately test soil resistivity in a diverse range of conditions.

There are four basic parameters that influence soil resistivity values

  1. Chemical content (soil type, constituents);
  2. Moisture content;
  3. Temperature; and
  4. Particle size and distribution.

The use of geotechnical data from the proposed testing area will often provide good background information and sanity check of test results.

Accurate soil resistivity for power system earthing

The Wenner or “four pin equal spacing” method is the preferred test regime used by LCS. Other methods include the Schlumberger or Driven Rod, and these have advantages in certain specific conditions.

Dr. Frank Wenner of the U.S. Bureau of Standards (now NIST) developed the theory behind this test in 1915. He showed that, if the electrode depth b is kept small compared to the distance between the electrodes a (b equal to one twentieth of a is the accepted upper limit), the following formula applies:

Soil resistivity Rho, is equal to 2 times Pi, times the spacing a, times the test meter resistance R, or
ρ = 2.π.a.R

In the Wenner method, it is important to understand how far apart test probes need to be in successive tests to provide an accurate soil profile as an input to the earthing design software.

The largest spacing (see “a” below) needs to be at least equal to the “zone of influence” of the earthing system being designed.  Click here for a sample test instruction showing recommended spacings.

The meter measures “apparent resistivity”, not the “actual” resistivity. The value obtained is a “weighted average” of the actual resistivity down to the depth (spacing) being tested. This raw data must be interpreted by software (e.g. CDEGS RESAP module) in order to determine the actual soil resistivity.

However for large, extended earthing systems (e.g. large power stations, utility scale solar PV farms) longer traverses are necessary for accurate earthing system analysis. Historically long traverses have not been done because their importance was not fully understood. As indicated above, the maximum spacing of a Wenner test should equal the power plant size, though this may not be practical for some larger sites.

With practicality in mind, the longer traverses should be 300-500 metres. If the test data has not stabilised at these spacings, they should be extended incrementally. For those larger sites, it is recommended to conduct several long traverses in different areas of the proposed plant area.

How many test points are required in a traverse set?

Experience has shown that the greater number of test points in a Wenner traverse, a higher accuracy soil model (multi-layer) will be developed by software. We recommend at least 10 data points in each traverse.

Software computations are more accurate if the data recorded does not have too many “gaps” between the spacings. The preferred ratio between spacings is 1.5. A reading taken at 5 metres, would need to be followed up by a 7.5 metre spacing, and preceded by at least 3.5 metres, in order to adhere to this 1.5 rule.

Soil resistivity test results and analysis

The use of sophisticated modelling software such as CDEGS’ RESAP module is able to provide accurate multi-layer soil profiles. A % error is provided with each software model.

Multi-layer software analysis is critical to providing accurate inputs to the design process. High/Low versus Low/High soil models give quite different results for an injected earth fault current.

Large variations in soil resistivity near the surface influence step and touch voltage performance. Whereas large variation deeper below the ground surface affect the transfer of earth potential rise to adjacent metalwork and services.

Using equivalent uniform soil resistivity and empirical equations from IEEE80 give large errors. Therefore simply “averaging” the raw test data results is not advisable.

Limitations of portable earth test meters

The maximum spacing able to be measured accurately is dependent on the test meter characteristics. Generally the power output characteristics of high-frequency soil resistivity meters are only adequate for maximum 50-60 metre spacings in the Wenner test. This is especially so in low resistivity soils. For greater test probe spacings, low-frequency soil resistivity test meters are necessary. These generate the required voltage to drive the signal through the soil at deep distances and detect weak signals, free of the usual error sources in high-frequency meters (e.g.induced voltage from the current injection leads).

High-Frequency soil resistance meters typically use a pulses operating at over 100Hz. These high-frequency meters do not generate high enough voltages for those long traverses and generally should not be used for probe spacings greater than about 50-60 metres. Also the high-frequency current injected into the outer probes induces noise in the voltage leads, which is difficult to filter out. This noise becomes greater than the measured voltage as the soil resistivity decreases and the test probe spacing increases.

Low-frequency meters generate pulses typically in the order of 0.5, 1.0 and 2.0 seconds per pulse. These are the preferred testers for soil resisitivty measurements, as they eliminate the induction problems typical of high-frequency test meters. However low-frequency meters are expensive. Depending on the maximum power output, low-frequency meters can take accurate measurements with very large probe spacings. This is because the electronics filtering in low-frequency meters are far superior.

Soil resistivity and corrosion

One of the main factors governing the rate at which corrosion occurs in buried earth systems is soil resistivity. The soil resistivity depends on the mechanical and chemical composition of the soil and the moisture content. The lower the soil resistivity the less metal will be required in the ground to achieve a given earth resistance, but the more corrosive the environment is to the electrodes.

More generally however, the properties of soil that impacts the corrosion rate of earth conductors are complex, but can be summarised by the following:

  • Soil resistivity – (inverse of conductivity) the lower the resistivity, the higher rate of corrosion.
  • Aeration – more air, less corrosion, drier soil reduces galvanic action (inversely proportional to particle size).
  • Moisture content or water retention – the more water, the more electrolyte, the more corrosion.
  • Dissolved salt content – means higher conductivity, and greater corrosivity.
  • Acidity or pH level – soils that test either end of the pH scale are highly corrosive to buried metals.
  • Presence of ionic species – halide ions (Chloride) and active bacteria (organic material fed by sulphates) produce acid soil environments.

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